In a previously known head-up display (HUD) apparatus, a bundle of rays of light is diffused by a screen member, and the bundle of rays of light, which is diffused by the screen member, is projected as a display image onto a projection surface of a vehicle (e.g., an automobile) to enable a viewer to see a virtual image of the display image.
A screen member of an HUD apparatus recited in, for example, JP2009-128659A, JP2010-145746A or JPH07-270711A, is an optical member, in which a plurality of micro-optical elements is arranged in a lattice pattern to transmit or reflect a bundle of rays of light that is incident on the respective micro-optical elements. The micro-optical elements are generally referred to as micro-lenses or micro-mirrors. In JP2009-128659A, each micro-optical element is configured into a shape of circle. In JP2010-145746A, each micro-optical element is configured into a shape of hexagon. Furthermore, in JPH07-270711A, each micro-optical element is configured into a shape of quadrangle (rectangle).
In JP2009-128659A, JP2010-145746A or JPH07-270711A, a surface of each of the micro-optical elements is formed as a curved surface to diffuse the bundle of rays of light, which is incident onto the curved surface, at a predetermined angle of diffusion. Therefore, the bundle of rays of light, which is outputted from the optical element at the predetermined angle of diffusion and is projected onto the projection surface, is guided to a predetermined range that is set around eyes of the viewer. At this time, the shape of the bundle of rays diffused by the optical element corresponds to the shape of the optical element, which is defined by the outer edges of the optical element. Thereby, the shape of the predetermined range, into which the bundle of rays is guided, corresponds to the shape of the optical element. Therefore, in this predetermined range, even when an eye point of the viewer is moved, the viewer can still visually recognize the display image. That is, this predetermined range is a viewing range, in which the viewer located in the vehicle can visually recognize the virtual image of the display image.
JPH07-270711A recites that an angle of diffusion of the diffused rays in a widthwise direction of each optical element and an angle of diffusion of the diffused rays in a height direction of the optical element can be individually controlled by changing a width and a height of the optical element.
The inventors of the present application have conducted an experiment by changing the width and the height of each optical element. As a result of this experiment, it has been found that when the width and the height of the optical element have a predetermined relationship, the virtual image of the display image, which is visually recognized by the viewer (occupant of the vehicle), is blurred.